Digital signal communications systems using satellites as the means for relaying the information are well known. Before being transmitted, the digital information is first subjected to a coding operation for error correction purposes (redundant check bits are added to the digital information). Also, the digital information is subjected to modulation to form channel symbols to increase the signal-to-noise ratio of the information and to increase the number of information bits conveyed by each channel symbol.
A typical example of a known system using Coded Phase Shift Keying (CPSK) as the coder/modulator is shown in FIG. 1. An information sequence of a known data rate (for example, in Mbps) is input to a serial-to-parallel converter 100, where the input information sequence is divided up into a number of parallel groups of information bits. In FIG. 1, the number of groups is equal to m. Then, these parallel groups of information bits are input to m individual encoders 101 (encoders C.sub.1 -C.sub.m). Then, the outputs of the individual encoders 101 are sent to a mapper 102 which maps the encoded information to create channel symbols for transmission over a communications channel. The symbols from mapper 102 are used by modulator 103 to provide a carrier having a different phase for each symbol, the selected phase being dependent upon the output of mapper 102.
Also well known, is the decoder/demodulator arrangement of FIG. 2, which is the counterpart of the encoder/modulator arrangement of FIG. 1. In FIG. 2, the received channel symbols (after PSK demodulation) are input to individual decoders 201, where the information is decoded in a well known fashion and sent to parallel- to-serial converter 202, where the information is recombined into a serial digital information data stream. In this way, the information sequence which was originally input to serial-to-parallel converter 100 of FIG. 1 is substantially recovered at the output of parallel-to-serial converter 202 of FIG. 2.
In the above-described FIGS. 1 and 2, the specific rates of the encoders/decoders are chosen in order to accommodate an expected information sequence bit rate. For example, in FIG. 1, if an information sequence bit rate of 100 Mbps is expected, the serial-to-parallel converter 100 could be set to divide the information sequence into, for example, m=10 parallel bit streams, each bit stream having 10 Mbps as a data rate. Then, at the parallel-to-serial converter 202 of FIG. 2 at the receive end, the information would be recombined into the original 100 Mbps data rate.
An alternative to sending information by satellite is the use of an optical network, for example, a fiber optic network. The synchronous optical network (SONET) is a family of interfaces for use primarily in optical networks. The SONET standard is designed to specify how optical signals would be transported between a number of different vendors' equipment and networks. The SONET standard, among several other specifications, provides an interface to broadband integrated digital networks (B-ISDN). B-ISDN provides broadband services, such as broadcast TV, high definition TV, transmission of database files at a high data rate, etc. The standard line bit rate of OC-3 (Optical Carrier Level 3) in the SONET hierarchy is 155.52 Mbps (R. Ballart and Y-C Ching, "SONET: Now It's the Standard Optical Network", IEEE Communications Magazine, March 1989.), which equals the standard bit rate for B-ISDN.
Given the high reliability of satellites, it is to the advantage of network operators to have available B-ISDN compatible economical links via the INTELSAT satellite system, by using only one 72 MHz transponder. Such satellite links can interconnect B-ISDN networks and, furthermore, can provide early introduction of B-ISDN prior to completion of the entire terrestrial network. Moreover, in optical fiber networks, satellites can also act as a "safety valve". That is, in the case of fiber failure network congestion, traffic can be bypassed through a satellite channel on a demand-assigned basis.
Current INTELSAT V/VA TDMA links use QPSK modulation, with a transmission rate of 60 Msymbol/s. The INTELSAT V/VA system has a transponder frequency spacing of 80 MHz, and a usable bandwidth of 72 Mhz per transponder. Channel symbol rates higher than 60 Msymbol/s can result in considerable degradation in the bit error rate (BER) performance, because of the intersymbol interference (ISI) generated by satellite multiplexing filters and adjacent channel interference (ACI) from the adjacent transponders. With a forward error correction coding (FEC) of rate 7/8, the bandwidth efficiency of the QPSK TDMA system is about 1.31 bits/s/Hz of the allocated bandwidth.
A recently developed coded octal phase shift keying (COPSK) modem, with a transmission rate of 60 Msymbol/s, supports an information rate of 140 Mbps, and has been field tested to demonstrate restoration of the TAT-8 fiber optical cable by satellite. The implemented 140 Mbps modem/codec consists of a 16-state trellis code of rate 7/9 and an octal PSK (OPSK) modem (F. Hemmati and R. Fang, "Low Complexity Coding Methods for High-Data-Rate Channels", Comsat Technical Review, Vol. 16, No. 2, Fall 1986, pp. 425-447). Bandwidth efficiency of the 140 Mbps COPSK system is 1.75 bits/s/Hz of the allocated bandwidth, which is an improvement of 33% over the bandwidth efficiency of the QPSK TDMA system.